1. www.postersession.com Results & Discussion Effects of cumulus clouds on microclimate and shoot-level photosynthetic gas exchange in Picea engelmannii and Abies lasiocarpa David Cook, Alyssa Heisler, Jasmine Jordan, Nicole Hughes High Point University, Department of Biology Acknowledgements This material is based upon work supported by the National Science Foundation Under Grant Number 1122064, awarded to Wake Forest University (PI William K. Smith) and Research Undergraduate Institution (RUI) High Point University (Co-PI Nicole M. Hughes). Consistent with the cloud-gap effect, cumulus clouds (Fig. 1A) corresponded with dramatic decrease in PAR during cloud-shade and significantly higher maximum PAR during cloud gaps (Fig 1B and D), as well as significant (2-3 fold) increases in diffuse PAR (Fig. 1C). Reductions in total PAR during cloud-shade corresponded with reductions in needle temperatures (Fig. 1E) and accordingly, leaf-to- air vapor pressure difference (LAVD) (Fig. 1G). Introduction Materials & Methods Cloud regimes at our study site (Medicine Bow Mountains, WY) are characterized by clear mornings followed by convective, cumuliform cloud formation and thunderstorms in the afternoon. The effects of cumulus clouds on plant physiology have not been fully characterized. Previous studies have shown that partly-cloudy conditions (i.e. clear skies punctuated by intermittent, cumulus cloud cover) can significantly increase sunlight intensity during cloud gaps due to summation of direct solar radiation and the additional diffuse light scattered/reflected by clouds. The effects of this increase in radiation (also known as the “cloud gap effect”) on species adapted to cloud regimes with frequent cumulus cloud cover has yet to be characterized. Objectives: Quantify the effects of cumulus clouds on microclimate and photosynthetic gas exchange in saplings of two conifer species, Picea englemannii and Abies lasiocarpa, at treeline (3210 m) in the Medicine Bow Mountains, Wyoming. Conclusions From these data we draw the following conclusions: - Cumulus solar-irradiance reflection does not significantly increase carbon gain in this system during cloud gaps. - Cloud-shade significantly reduces photosynthesis, but also has a cooling effect on needles and shoots, which results in significant reductions in evapotranspiration and increases in water use efficiency, the effects of which are sustained even during cloud-gaps. Our field site is located at the alpine-treeline ecotone (elevation 3210 m) in the Snowy Range of the Medicine Bow Mountains in southern Wyoming, USA. Photosynthetic gas exchange and microclimatic measurements were taken on Picea engelmannii and Abies lasiocarpa saplings under clear and partly-cloudy conditions during June-September 2012. Instruments used: Li-Cor LI-6400XT equipped with a conifer needle chamber, LI-190 PAR sensor, Raynger ST80 infrared thermometer, BF5 sunshine sensor, and HOBO data loggers. Shoot photosynthetic gas exchange was measured at 3hr intervals between 0900 and 1700 hr. Mean photosynthesis, conductance, and evapotranspiration under clear, cloud- shade, and cloud-gap intervals were compared each month using a one way ANOVA with Tukey follow-up test. An additional experiment was conducted to synchronously measure photosynthetic gas exchange, chlorophyll fluorescence, and diffuse and direct PAR at 30 second intervals before, during, and after cumulus cloud cover. Cumulus cloud shade resulted in significantly reduced photosynthesis and evapotranspiration in both species (Fig. 2), most likely as a direct result of reduced PAR and needle temperatures, respectively (Fig. 1). No significant difference in A was observed between cloud-gaps and clear sky conditions in either species, suggesting that increased diffuse light from cumulus solar irradiance reflection does not correspond with significant increases in photosynthesis. However, reduced evapotranspiration paired with constant A resulted in higher instantaneous water use efficiency during cloud gaps, which may translate into reduced water stress. Figure 3 shows these responses on the scale of 30 second intervals, as individual cumulus clouds pass overhead (shaded columns). DEFGDEFG Figure 1. Effects of cumulus clouds (A) on diurnal total (B) and diffuse (C) PAR on a sample cloudy (dotted line) versus clear(solid) day. Right column depicts effects of an individual cumulus cloud on various micrometeorological parameters. Figure 2. Comparative mean gas exchange parameters for P. engelmannii (n=21-53 ±SE) and A. lasiocarpa (n= 13-24 ±SE) under clear-sky conditions compared to measurements taken at the same time during the same month under partly cloudy conditions, both under cloud-shade, and between clouds (cloud-gaps). Letters above bars represent results from Tukey test; bars with shared letters are not significantly different. Statistical results for P. engelmannii are represented by capital letters, while results for A. lasiocarpa by lower case letters. Figure 3. Synchronized total and diffuse PAR, photosynthesis (A), conductance (g), evapotranspiration (E), and instantaneous water use efficiency (WUE) measurements taken every 30 sec over 30 minute time intervals. All data taken on the same A. lasiocarpa shoot between 1325-1400. Data shown are from one completely clear afternoon (14- Jun, 2013; left panels) and one partly cloudy afternoon (15-Jun, 2013; right panels). Shading corresponds with the presence of cloud-shade.